1. Technical Field
The present invention relates to an optical semiconductor device package, and more particularly, to an optical semiconductor device package which permits a cooling device for cooling an optical semiconductor device to be appropriately solder-joined thereto and an optical semiconductor device module having a package of this kind.
2. Related Art
In an optical communication system, optical semiconductor device modules are employed, which comprise a package and an optical semiconductor device accommodated therein for receiving and/or transmitting an optical signal. Although the optical semiconductor device constituting such a module is requested to produce a high power, it generates heat and is raised in temperature when operated at a high power level, so that its operation may become unstable. A similar problem can occur if the optical semiconductor device module is employed under high temperature circumstances. Thus, the optical semiconductor device module is generally provided with a cooling device for cooling the optical semiconductor device, thereby stabilizing the operation of the optical semiconductor device.
As this kind of optical semiconductor device module, a transmitter module shown in FIG. 1 is known. This transmitter module comprises, as shown in FIGS. 1 and 2, an optical semiconductor device package comprised of a metal bottom plate 1 and a metal frame 2 joined to an upper periphery of the bottom plate. The metal bottom plate 1 is ordinarily made of a Cu-W alloy and has a surface thereof plated with Au. The metal bottom plate 1 forms a package housing in conjunction with the metal frame 2 and a seal ring 3 joined to an upper face of the metal frame. The package housing accommodates therein module components such as an optical semiconductor device and a cooling device. The metal frame 2 has a right wall to which a hollow cylindrical window frame 5 is joined, and an optical fiber 20 is drawn out to the outside through a glass or sapphire window joined to the window frame 5.
In the illustrated optical semiconductor device module, a Peltier device 10 serving as the cooling device is solder-joined to an upper face of the metal bottom plate 1 in a state it is disposed in a recess 1a formed on that face, and a base which is excellent in heat and electric conductivity is solder-joined to the upper face or cooling face 10a of the Peltier device 10. Furthermore, module components such as laser diode carrier 12, photodiode carrier 13, first lens 14, optical isolator 15 are solder-joined to the base 11, with these elements aligned with one another. Then, a laser diode (LD) 16 and a thermistor (not shown) for monitoring the temperature of the LD are solder-joined to the LD carrier 12, and a photodiode (PD) 17 for monitoring an LD light signal is solder-joined to the PD carrier 13. A second lens 19 is disposed inside the window frame 9. Leads 18 are supported by electrical signal input/output sections 7 of a ceramic material which are joined to notches formed in front and rear walls of the metal frame 2, respectively.
After the module components are received in the package housing, lead terminals of the LD 16 and respective one ends of the leads 18 are electrically connected to each other by means of wires, not shown, to thereby make it possible to transfer electric signals between the LD 16 and external devices through the leads 18.
Finally, a metal lid 4 is seal-welded to the metal frame 2 through the seal ring 3, whereby the fabrication of the optical semiconductor device module is completed. The metal lid 4 may be directly seal-welded to the metal frame 2 without using the seal ring 3.
The Peltier device 10 comprises, for instance, two insulating layers and P-type and N-type thermoelectric conversion devices that are alternatively arranged between the insulating layers and electrically connected in series with one another. The two insulating layers have functions of generating and absorbing heat at their surfaces, respectively, when a DC voltage is applied to the thermoelectric conversion devices. Outputs from the thermistor and the PD are employed for temperature control and constant optical-output control in the optical semiconductor device module, respectively.
With the optical semiconductor device module having the above construction, heat generated by the LD 16 is transferred to the cooling face 10a of the Peltier device 10 through the LD carrier 12 and the base 11 and then dissipated to the outside through the heat generating face 10b of the Peltier device 10 and the metal bottom plate 1 serving as a heat sink, if the Peltier device 10 is appropriately solder-welded to the metal bottom plate 1, thereby preventing the LD 16 from becoming excessively high in temperature. However, an improper soldered joint may be formed between the metal bottom plate 1 and the Peltier device 10 for the following reasons.
In conventional solder joining processes, the cooling device such as a Peltier device 10 is placed on a solder foil of 20 to 50 xcexcm thick disposed on the metal bottom plate 1, and then the solder foil is heated to melt while pressing the Peltier device 10 by means of a weight, not shown, placed thereon. As the solder foil is heated and raised in temperature, the plated Au on the surface of the metal bottom plate 1 diffuses into the solder foil, so that the melting point of the Au diffusion part of the solder foil becomes high. The high melting point part cannot melt during the process of solder-joining the metal bottom plate 1 and the Peltier device 10, and hinders the spread and wetting of molten solder between the opposite faces of the metal bottom plate 1 and the Peltier device 10 to thereby form air spaces or voids therebetween. As a result, unmelted parts of the solder foil and voids remain between the metal bottom plate 1 and the Peltier device 10 after completion of the solder joining process.
Since the adhesion between the metal bottom plate 1 and the Peltier device 10 is lowered by the unmelted parts and the voids, there occurs a deterioration in heat transfer between the metal bottom plate 1 and the heat generating face 10b of the Peltier device 10 whose cooling face 10a receives heat generated by the LD 16. Thus, the Peltier device 10 cannot fully dissipate heat and is deteriorated in its cooling ability, so that the LD 16 is raised in temperature and becomes unstable in operation. If unmelted solder or a void is present between the metal bottom plate 1 and the Peltier device 10, the Peltier device 10 and the LD 16 are inclined relative to the metal bottom plate 1 so that the optical axis of the LD 16 goes out of alignment, causing improper optical signal transmission in the optical semiconductor device module.
To prevent the solder foil from increasing in melting point attributable to diffusion of Au thereinto, if a countermeasure such as to increase the thickness of the solder foil, i.e., the solder joint layer between the metal bottom plate 1 and the Peltier device 10 is taken, a tilt of the Peltier device 10 relative to the metal bottom plate 1 is liable to become large, and if another countermeasure such as to decrease an amount of Sn in the solder foil is taken, the adhesion between the metal bottom plate 1 and the Peltier device 10 is lowered since the melting point of the solder foil excessively decreases at parts other than Au diffusion parts of the solder foil. Thus, these countermeasures are not useful to attain an improved solder joint between the metal bottom plate and the Peltier device in the optical semiconductor device module and at the same time reduce the tilt of the Peltier device relative to the metal bottom plate.
An object of the present invention is to provide an optical semiconductor device package permitting a cooling device for cooling an optical semiconductor device to be properly solder-joined thereto, and an optical semiconductor device module having this kind of package.
According to one aspect of the present invention, there is provided an optical semiconductor device package which includes a metal bottom plate having an inner face thereof including a solder joint area for solder joint between the metal bottom plate and a cooling device for cooling an optical semiconductor device, a metal frame joined to a peripheral portion of the inner face of the metal bottom plate and cooperating with the metal bottom plate to define an accommodation space for accommodating therein the optical semiconductor device and the cooling device, a window frame formed integrally with the metal frame and employed for optical signal transmission between the optical semiconductor device and an external device, and an electric signal input/output section formed integrally with the metal frame and employed for electric signal transmission between the optical semiconductor device and an external device. The optical semiconductor device package of this invention has an improvement in that at least one groove permitting molten solder to flow therein is formed in at least part of the solder joint area on the inner face of the metal bottom plate.
With the package of the present invention, when solder is heated to melt in the groove formed on the inner face of the metal bottom plate in order to produce a solder joint between the metal bottom plate and the cooling device, molten solder flows in the groove and penetrates into between opposite faces of and permeates into surface layers of the metal bottom platen and the cooling device.
In making a solder joint between the cooling device and the metal bottom plate of the package of this invention, plated Au on the surface of the metal bottom plate diffuses into solder which is being subject to heating, as in the case of the conventional package. However, an amount of solder useable for the formation of the solder joint for the package of this invention is larger by an amount flowing in the groove than that for the conventional package, and hence the concentration of Au in the solder joint layer becomes smaller, if the solder joint layers to be formed between the metal bottom plate and the cooling device are identical in thickness for both the packages. As a result, occurrences of unmelted solder and voids in the solder joint layer due to an increased melting point of solder are prevented or suppressed, and hence the adhesion between the metal bottom plate and the cooling device through the solder joint layer is enhanced, whereby the cooling device adequately achieves the effect of cooling the optical semiconductor device. In addition, since the thickness of the solder joint layer and the amount of Sn in solder can be made proper, the tilt of the cooling device relative to the metal bottom plate can be eliminated or suppressed and a proper solder joint can be attained.
In the present invention, preferably, a plurality of grooves permitting molten solder to flow therein are formed in at least the solder joint area on the inner face of the metal bottom plate. More preferably, the plurality of grooves include a group of grooves extending in substantially the same direction and spaced from one another, and one or more grooves extending such as to cross the group of grooves. With this construction, a solder joint layer can be uniformly formed between the metal bottom plate and the cooling device.
Preferably, the at least one groove or at least one of the plurality of grooves is comprised of a central portion and opposite end portions communicating therewith. At least the central portion extends, as viewed in plan, in the solder joint area on the inner face of the metal bottom plate, and at least one end portion extends off the solder joint area, as viewed in plan, on the inner face of the metal bottom plate. With this construction, solder can be placed in one end portion of the groove, facilitating an operation of forming a solder joint between the metal bottom plate and the cooling device.
More preferably, the groove has its opposite end portions extending off the solder joint area on the inner face of the metal bottom plate. With this structure, air does not remain in the groove during the solder joining operation, so that the solder joint layer can be rapidly formed and the formation of voids in the solder joint layer can be prevented.
Preferably, each groove has a depth of 5 to 120 xcexcm, more preferably, 10 to 100 xcexcm. With this arrangement, there is a low possibility that the ability of molten solder to flow in the groove is lowered by an increased viscous resistance of molten solder or by a reduced capillary action of the groove.
Preferably, the area ratio of the groove to a surface of the cooling device on the side facing the metal bottom plate is equal to or larger than 30%. More preferably, the area ratio is equal to or larger than 50%. With this arrangement, molten solder rapidly flows in the groove.
According to another aspect of this invention, there is provided an optical semiconductor device module which comprises the above-mentioned optical semiconductor device package of this invention, an optical semiconductor device accommodated in the package, and a cooling device solder-joined to the solder joint area on the inner face of the metal bottom plate of the package and operable to cool the optical semiconductor device.
With the optical semiconductor device module of this invention, the metal bottom plate and the cooling device are properly solder-joined to each other to provide enhanced adhesion therebetween, so that heat generated by the optical semiconductor device is adequately transferred through the cooling device to the metal bottom plate and dissipated therefrom to the outside, whereby the operation of the optical semiconductor device is stabilized. Since there is a low possibility that the cooling device is inclined relative to the metal bottom plate, a misalignment of the optical axis of the optical semiconductor device and a resulting improper optical signal transmission may not be caused.